Dog clutches generally possess two clutch members which can rotate about an axis of rotation, being provided on their mutually facing end surfaces with complementary engaging contours or tooth systems. The engaging contours or tooth systems are each composed of one or more rows of claws or teeth, being separated from neighboring teeth by tooth gaps and adapted to each other such that, when the clutch is engaged, the teeth of each clutch member reach into the gaps between the teeth of the other clutch member and produce a torque-resistant and form-fitting connection between the driving clutch member and the driven clutch member. One of the two clutch members is usually stationary in the axial direction of the axis of rotation, while the other clutch member can be moved back and forth by a positioning device in the axial direction of the axis of rotation, so that it can be brought into a form-fitting tooth engagement with the other clutch member to engage the clutch, or can be moved away from the other clutch member to disengage the clutch so that the teeth of the driving clutch member can rotate past the driven clutch member without making contact with its teeth.
Upon engaging or closing of dog clutches, however, it may happen that the teeth of the two clutch members to not stand opposite or abut against each other in pairs, so that they do not enter into the tooth gaps of the other clutch member. This means that the oppositely situated teeth of the two clutch members are only pressed against each other with friction locking, but no form-fitting connection is produced and thus neither is there any complete flow of force between the two clutch members.
A secure engaging or closing of dog clutches in the drive train of motor vehicles is generally ensured by a synchronizing mechanism, which adapts the rotary speed of the driven clutch member prior to the engaging of the clutch to the rotational speed of the driving clutch member so that the two rotational speeds differ slightly. In this way, when the two clutch members approach each other, the teeth of the two clutch members rotate slowly past each other and upon further approaching they enter into the gaps between the teeth of the other clutch member.
But since synchronizing mechanisms cause a not inconsiderable additional expense, one looks for possibilities of avoiding this expense, so that even without a synchronizing mechanism it is ensured that the clutch is in fact engaged or closed and that the clutch members are not standing “tooth on tooth”.
In motor vehicles of the applicant with a disconnectable all-wheel drive, a dog clutch without a synchronizing mechanism is used in the drive train in order to switch on the switchable axle by engaging the dog clutch or to switch it off by disengaging the dog clutch. But since, due to the lack of a synchronizing mechanism, when the motor vehicle is standing still, the teeth of the one clutch member may either lie against the teeth gaps or the teeth of the other clutch member, on the one hand there should not be too much torque delivered when the clutch is engaged in a stationary vehicle, as this might lead to wear and tear if the two clutch members are still standing “tooth on tooth”. On the other hand, however, adequate torque should be delivered to make possible a turning of the two clutch members in relation to each other in the case of a “tooth on tooth” position and to distribute torque on both axles when first driving off. This compromise is seen as a disadvantage.